The short answer: Dentists find cavities with a careful visual exam, backed up by X-rays for the areas they can’t see, such as between the teeth. X-rays are good at spotting cavities that have reached the inner layer of the tooth but miss many of the earliest enamel lesions, and laser and light-based devices are add-ons whose accuracy varies. For gum disease, a dentist or hygienist measures the pockets around each tooth with a thin probe (healthy pockets are 1 to 3 mm deep), checks for bleeding and uses X-rays to look for bone loss. Finding problems early matters because early decay can often be treated without a drill and gingivitis can be reversed.
Untreated tooth decay in permanent teeth is the most common health condition in the world, and severe gum disease affects more than 1 billion people (World Health Organization, 2025). Both can progress quietly: tooth decay may not cause any symptoms at first (NHS, 2025), which is why dentists look for problems at check-ups rather than waiting for pain.
This guide explains how dentists check for cavities and gum disease, how accurate each method is, whether dental X-rays are safe, what newer tools such as laser devices and AI add, and how early problems are treated.
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How do dentists check for cavities?
The visual exam
The cornerstone of finding cavities is a visual and tactile exam (Macey et al., 2021b), in which the dentist looks closely at each tooth surface for signs of decay such as white, brown or black spots (NHS, 2025). A probe can be used to feel the surface, but gently, because pressing hard can damage weakened enamel (American Dental Association, 2023).
How accurate is looking? A Cochrane review of 67 studies found that visual classification systems, such as the International Caries Detection and Assessment System (ICDAS), correctly identified 86% of early enamel cavities and correctly cleared 77% of healthy surfaces. Results varied widely between studies and the certainty of the evidence was low, partly because many studies were done on extracted teeth (Macey et al., 2021a).
Dental X-rays
X-rays, especially bitewings, show areas the eye can’t reach, such as the surfaces between back teeth. A systematic review of 117 studies found that X-rays are highly accurate for cavities between teeth that have already formed a hole, and suitable for finding decay that has reached the dentin, the layer under the enamel, although they still miss some of it. They were much less sensitive for the earliest lesions, although they rarely flagged healthy teeth by mistake (Schwendicke et al., 2015).
A Cochrane review focused on those earliest, enamel-only lesions found that X-rays and other imaging caught only about 47% of them while correctly clearing about 88% of healthy surfaces, based on low-certainty evidence (Walsh et al., 2021). That is why X-rays support the visual exam rather than replace it.
Are dental X-rays safe, and how often do you need them?
Dental imaging accounts for less than 1% of the radiation people receive from medical imaging (American Dental Association, 2026). Still, every X-ray should be justified: 2024 expert-panel recommendations published in the Journal of the American Dental Association say dentists should examine you first and take images only when there is a clear clinical benefit, keeping exposure as low as reasonably achievable (Benavides et al., 2024; American Dental Association, 2026).
There is no one-size-fits-all schedule. How often you need X-rays depends on your oral health, age, risk of disease and any symptoms (American Dental Association, 2026). The 2024 recommendations also changed something patients notice: thyroid collars are no longer recommended for any dental imaging, because shields such as collars and lead aprons can block the X-ray beam and lead to repeat images. Restricting the beam to the area being imaged protects patients better (American Dental Association, 2026). Digital X-rays can reduce exposure compared with film, and studies confirm the safety of dental treatment during pregnancy (American Dental Association, 2026).
Do laser and light-based cavity detectors work?
Some practices use devices that shine light on the teeth to help find early decay. Laser fluorescence devices, such as DIAGNOdent, measure how much a tooth surface fluoresces, which changes with decay. A Cochrane review estimated that they miss about 3 in 10 early enamel lesions and wrongly flag about 1 in 5 healthy surfaces, and it rated the certainty of the evidence as low (Macey et al., 2020).
Transillumination devices shine light through the tooth so that decay shows up as a shadow. Across all such devices, a Cochrane review found they caught 75% of early enamel cavities and correctly cleared 87% of healthy surfaces. Near-infrared devices caught only 58%; optical coherence tomography did much better, at 94%, but it is not yet available in general dental practice (Macey et al., 2021b). These tools can help in uncertain cases, but none replaces the dentist’s exam.
What about intraoral cameras and AI?
Intraoral cameras let you see close-up images of your own teeth on a screen. Research on them has focused mainly on helping patients understand what the dentist sees and follow advice (Pentapati & Siddiq, 2019).
Artificial intelligence software that reads X-rays is increasingly common. A 2026 meta-analysis found that AI caught 84% of cavities between the teeth and correctly cleared 93% of healthy areas, although the certainty of the evidence was low (Guo et al., 2026). In a randomized trial, dentists who read bitewing X-rays with AI support caught more cavities, raising their sensitivity from 72% to 81% (Mertens et al., 2021). Our article on how AI is used in dentistry covers what has been cleared by the FDA and its limits.
How do dentists check for gum disease?
The key test is probing. A dentist or hygienist gently slides a thin, marked probe into the space between each tooth and the gum to measure the pocket depth; healthy pockets measure between 1 and 3 millimeters (National Institute of Dental and Craniofacial Research, 2026). They also note whether the gums bleed when probed, which is the main sign used to define gingivitis (Chapple et al., 2018), check whether any teeth are loose and take X-rays to look for bone loss (National Institute of Dental and Craniofacial Research, 2026).
If periodontitis is found, the amount of attachment and bone lost, along with factors such as smoking and diabetes, is used to assign a stage (I to IV) and a grade (A to C), which guide treatment (Tonetti et al., 2018). Our guide to gum disease and bleeding gums explains the stages and treatment in detail.
Can early cavities and gum disease be reversed?
Often, if they are caught early. An American Dental Association guideline recommends non-drilling treatments, such as fluoride varnish, sealants and resin infiltration, for many early cavities that haven’t formed a hole (Slayton et al., 2018). In studies of children, professionally applied fluoride varnish remineralized about 64% of early enamel lesions (Gao et al., 2016) and reduced new decay in permanent teeth by about 43% (Marinho et al., 2013), and sealants on the permanent molars of children and teenagers prevent decay compared with no sealant (Ahovuo-Saloranta et al., 2017). See can a cavity heal on its own? and our guide to minimally invasive dentistry for the options.
For the gums, gingivitis can be reversed with daily brushing and cleaning between the teeth (Chapple et al., 2015). Periodontitis can be controlled with deep cleaning below the gumline: a systematic review found that this reduced pocket depth by about 1.4 mm on average and closed about three-quarters of the pockets treated (Suvan et al., 2020).
How often should you have a dental check-up?
It depends on your risk. A Cochrane review found little to no difference in outcomes between 6-month check-ups and intervals tailored to each person’s risk for adults who attend regularly, based on high-certainty evidence (Fee et al., 2020). Your dentist should set your interval based on your history of decay and gum disease. For what to expect between cleanings, see why yellow film comes back after a dental cleaning.
What it means for you
If your dentist recommends X-rays
It is reasonable to ask why they are needed now and what the dentist is looking for. Current guidance is that X-rays should be based on your individual needs, not a fixed schedule (American Dental Association, 2026).
If you’re told you have an early cavity
Ask whether it has broken through the enamel and whether it can be treated without a filling, for example with fluoride varnish or a sealant, and monitored at your next visit (Slayton et al., 2018).
If your gum pockets are 4 mm or deeper
Pockets deeper than 3 mm can be a sign of gum disease (National Institute of Dental and Craniofacial Research, 2026). Ask whether you have gingivitis or periodontitis, what the next step is, and when the pockets will be measured again.
Questions to ask your dentist
- How often do I need bitewing X-rays, given my risk?
- Is this spot an early cavity that we can watch and treat without drilling?
- What were my pocket depths, and did any sites bleed?
- Do you use any additional detection tools, and how do they change what you recommend?
- How often should I come back for check-ups?
When to see a dentist
Book a dental visit if (NHS, 2025; NHS, 2026):
- You have toothache or a sharp pain when eating or drinking hot, cold or sweet things
- You notice white, brown or black spots on a tooth
- Your gums are painful, swollen or bleed when you brush
- You have bad breath that doesn’t go away
For a knocked-out or broken tooth, facial swelling or other urgent problems, see what to do in a dental emergency.
The bottom line
Dentists find cavities mainly by looking carefully, with X-rays for the areas they can’t see, and they find gum disease by measuring pockets, checking for bleeding and looking for bone loss on X-rays. X-rays miss many of the earliest lesions, and laser, light-based and AI tools are useful add-ons with variable accuracy. Regular check-ups at the interval your dentist recommends give the best chance of catching problems while they can still be treated without a drill (Macey et al., 2021a; Schwendicke et al., 2015; Fee et al., 2020).
References
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References checked October 2026 · Editorial policy · Report a correction
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